Dual Character of Excited Radical Anions in Aromatic Diimide Bis(radical anion)s: Donor or Acceptor?

Intramolecular electron transfer (ET) processes in the excited aromatic diimide bis­(radical anion)­s (ADI^•–*–ADI′^•–) were systematically investigated by applying femtosecond laser flash photolysis to bis­(radical anion)­s of naphthalenediimide (NDI) and perylenediimide (PDI), including NDI^•––m-NDI^•–, NDI^•––p-NDI^•–, PDI^•––m-PDI^•–, and NDI^•––m-PDI^•– (m and p indicate the substitution positions). The excitation of NDI^•––m-NDI^•– and NDI^•––p-NDI^•– initiated disproportionation reactions generating NDI and NDI^2– with different ET rate constants. For the first time, the dual characteristics of ADI^•–* were confirmed upon selective excitation of NDI^•––m-PDI^•–: NDI^•–* was unambiguously demonstrated to function as an electron donor in NDI^•–*–m-PDI^•–, whereas PDI^•–* acted as an electron acceptor in NDI^•––m-PDI^•–* because of the energetically preferable production of NDI–m-PDI^2–. The relationship between the ET rate constants and driving forces in ADI^•–*–ADI′^•– can be reasonably analyzed by using the Marcus theory. The current findings provide a new viewpoint regarding the bipolaron-generating nature of ADI^•–*–ADI′^•– and facilitate simulating various types of photocarrier migration in the densely charged regions of homo- and heterogeneous n-type semiconductor materials upon irradiation.